Modern geophysical exploration techniques include both land-based and marine seismic surveys. In marine surveys, a seismic research vessel typically tows a source such as an airgun array, which periodically emits acoustic pulses generated by collapsing air bubbles. The acoustic waves propagate through the water column and penetrate the seabed or ocean floor, where they are reflected from boundaries between subsurface geological formations. The reflected acoustic energy is detected by an array of seismic sensors or receivers, which generate seismic sensor data that can be processed to reconstruct the reflected wavefield and generate images of the corresponding subsurface geology.
Typically, the seismic receivers are distributed along a series of streamer lines towed behind the seismic vessel, or deployed directly onto the seabed along an ocean-bottom cable. Receivers can also be deployed as an array of individual, autonomous sensor nodes.
Within the water column, acoustic energy is substantially characterized by the propagation of pressure-type acoustic waves (P-waves). Thus, towed seismic streamer arrays traditionally utilize pressure-sensitive receivers such as hydrophones. The subsurface wavefield, on the other hand, includes both pressure waves and shear waves (S-waves), in addition to more complex wavefield contributions. Modern ocean-bottom seismic systems thus employ motion-sensitive devices such as geophones and accelerometers as well, for example in a sensor subarray with a combination of hydrophone and multi-axis geophone components, sensitive to both differential pressure and motion (velocity or acceleration) along three orthogonal axes.
In this more general approach, the pressure and shear wave contributions are combined to more accurately reproduce the full seismic wavefield, and to generate more complete images of the subsurface geology. Similar techniques can also be applied in land-based surveys, where both pressure and shear wave data are also available.
In order to accurately track and log the substantial quantities of seismic sensor data required to achieve these results, precision clock systems are typically provided, along with local data processing and storage components, a power supply, and an interface configured for control and data communications. As each of these components increase in data capacity, there is an ongoing need for improved seismic imaging techniques adapted to handle the correspondingly greater data flow.